[kcmkwin/kwintabbox] Use a more appropriate icon

Summary:
The current icon (`window-duplicate`) is semantically incorrect and visually disconnected from what it actually does.

This one is much more appropriate on both counts.

Test Plan: {F6792003, size=full}

Reviewers: #vdg, #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20801